1. Field of the Invention
The present invention relates to a multi-layer piezoelectric laminate including a ceramic conductor and a piezoelectric ceramic, and more particularly to such a multi-layer piezoelectric laminate wherein the ceramic conductor is zinc oxide.
2. Related Art
In actuator systems, researchers try to maximize the generated force and displacement while maintaining a low operating voltage. Multi-layer stacks achieve large forces and keep the operating voltage low, but are limited in displacement. Inherent materials properties of piezoelectrics and electrostrictives have strains that are limited to a few tenths of a percent. Flextensional devices, those that bend like a bimetallic strip, sacrifice the generated force and amplify the strain by utilizing an electromechanical gradient. Low operating voltages with flextensional devices can be obtained by making the actuator thin, but at the expense of decreasing the load bearing capabilities.
The classic example of a flextensional device is the unimorph, shown with two typical support structures in FIG. 1. Consisting of a metal shim bonded to a piezoelectric, displacement occurs due to the electric field inducing a non-uniform lateral stress field. Bonding oppositely poled piezoelectric plates together, commonly called bimorphs, further enhances the bending strain by allowing each half to contribute to the generated moment.
A modified version of the unimorph, called the monomorph, is comprised of one semiconductive piezoelectric ceramic plate. Removal of the difficult and laborious tasks of cutting, polishing, and bonding is a processing advantage of the monomorph over the unimorph. The addition of (K.sub.1/2 Bi.sub.1/2)ZrO.sub.3 to lead zirconate titanate or barium titanate converts the normally insulating piezoelectric into a semiconductive piezoelectric. Bending occurs due to the non-uniform electric field distribution that arises from the semiconductor-metal interface under an applied voltage.
The Rainbow is another modification of the unimorph. Once again, the bending stress is achieved with a variation in the electromechanical properties across the thickness of the device. Rather than bond dissimilar materials together, one side of the lead oxide based ferroelectric disc is chemically reduced with respect to oxygen. The reduction process converts a portion of the ferroelectric into a ceramic metal composite. A complicated composite structure of lead metal and reduced ferroelectric ceramic is created. Effectively, the ceramic-metal portion replaces the non-piezoelectric in a standard unimorph.
Another variation of the unimorph is made with thin tapes of PZT powder doped with varying amounts of zinc borate. Stacking the undoped PZT tapes with doped tapes creates a gradient in resistivity. During the sintering process, the zinc ion diffuses through the sample to yield a nearly linear variation in zinc borate concentration. Regions in the actuator that are more conductive will not obtain the same extent of poling as the undoped region. Effectively, a variation in the magnitude of poling is obtained.
Efforts in the past to produce devices having strong bonds between the conductor and the piezoelectric, which may allow such devices to be miniaturized, include:
U.S. Pat. No. 3,676,722, outlines the structure for bimorph or monomorph benders. A bimorph bender is comprised of two piezoelectric wafers with a center vane of conductive material and a monomorph is comprised of a piezoelectric bonded to a conductive material bonded to a piezoelectrically inactive material. When a field is applied the piezoelectric will change shape and induce a bending moment. The attachments of each section are done with adhesive or metal solder.
U.S. Pat. No. 4,862,029, describes a similar device as above. The piezoelectric material is doped with K(Nb, Ta)O.sub.3 to alter the electrical properties of the piezoelectric. The mixture is pressed into a disc or plate shape and sintered to create a semiconductive piezoelectric. Without this processing step, the piezoelectric will normally be insulating. When electrodes are attached and an electric field is applied, a non-uniform potential is created at the semiconductive-metal interface. This results in non-uniform strain through the ceramic plate inducing a bending stress.
U.S. Pat. No. 5,471,721, describes a modification of the above inventions. One side of a monolithic lead oxide-based piezoelectric plate is chemically reduced with respect to oxygen. This portion of the ceramic is converted into a ceramic-metal composite that ceases to have piezoelectric properties. The chemical reduction has made this plat into a device that behaves as the invention outlined in U.S. Pat. No. 3,676,722.
U.S. Pat. No. 5,502,345, describes an actuator that has one region with lower resistivity in contact with a second region of higher resistivity. There is no seam where the said regions are in contact with each other. The device is similar to the one outlined in U.S. Pat. No. 4,862,029, and rather than doping with K(Nb, Ta)O.sub.3, they dope with iron oxide or zinc borate. Doping a lead-oxide based piezoelectric with iron oxide or zinc borate at a concentration of 0.5 to 3-weight % will increase the conductivity of the ceramic. Layering a doped ceramic with an undoped ceramic and subsequent sintering of the laminate yields the desired device. The doped portion is more conductive than the undoped portion. Diffusion of the doping agents across the interface during the sintering step removes any trace of the interface.
The present invention is different from the above mentioned inventions in that the present invention layers a ceramic (zinc oxide) with a piezoelectric ceramic (lead zirconate-titanate or PZT). The two different ceramics are stacked and sintered simultaneously resulting in a laminated monolith. The interface between the zinc oxide and PZT is sharp and remains after sintering. The interface thickness is about 20 microns wide. Silver electrodes are applied to two opposing surfaces parallel to the zinc oxide--PZT interface. An electric field is applied. Upon reaching a field around 50 to 70 V/mm thickness of zinc oxide, the conductivity of the zinc oxide dramatically increases. Essentially, the zinc oxide behaves like shim of metal. However, this device is completely ceramic and needs no post-sintering processing steps to join the piezoelectric to the non-piezoelectric.
None of these prior efforts teach or suggest a multilayer piezoelectric laminate having a piezoelectric ceramic and a ceramic conductor. Further, none teach or suggest the use of zinc oxide for the ceramic conductor.